Turns out up through 13499 (approximately) the speakers were indeed in the center console on either side of the transmission tunnel - which actually has scoop-outs in the metal to accommodate them. As of 13500 the speakers were moved to the door panels (but the scooop-outs int he transmission tunnel remained).

On the central console the speaker grilles were round. On the doors they were "rounded squares". I just got a pic from Frank Schwartz of the door grilles on his car and they are the original speaker grilles.

So now I can move from being sure I know what they look like, to finding a pair! Anybody out there have some spares they would be willing to part with?